1. Field of the Invention
This invention relates to novel composite titania pigments and their manufacture. In particular, the invention relates to novel off-white pigment compositions obtained from by-products (heretofore considered to be wastes) of the beneficiation of sedimentary kaolin clay to remove colored iron-bearing titaniferous impurities normally associated with the clay.
2. Prior Art
The usual pigmentary grades of titanium dioxide are relatively expensive pigments which are widely used to opacify a variety of systems such as plastics and paints. Conventional pigment grades of titanium dioxide contain over 88 percent TiO.sub.2, in some cases over 98 percent TiO.sub.2, and have a bright white color. These pigments may be obtained from ilmenite by the so-called "sulfate" process or, more recently, from rutile by the so-called "chloride" process. Typically pigmentary grades of titania have a brightness of 95 percent or above and have low abrasiveness, for example less than 50 mg. as measured by the well-known Valley Abrasion method. Average particle size is usually in the range of 0.2 to 0.4 micron. The high brightness, fine particle size and low abrasiveness of pigmentary titania is essential in many of the important commercial uses. A relatively inexpensive commercial titania product is obtained by extracting iron from ilmenite ores. Reprsentative products of this type, referred to as "upgraded ilmenites," analyze about 94 percent TiO.sub.2 and have a brightness of about 38 to 40 percent. The upgraded ilmenite has a buff color and is useful as an opacifier in certain systems in which the whiteness of conventional pigmentary titanium dioxide is not necessary. However, the product is a relatively coarse material that is difficult to grind. Even when fluid energy equipment is used, the average particle size exceeds 2 microns and the abrasion index is above 500 mg. This precludes use of upgraded ilmenite as an opacifier in many paint and plastic systems in which the white color of conventional titanium dioxide minerals is not essential.
Naturally-occurring kaolin clay crudes, such as those present in deposits in Georgia, contain discrete iron-bearing titania impurities which detract from the value of the clay as paper coating pigments. These impurities impart an undesirable yellow or orange tinge to the clay even when present at low concentrations, typically 1 percent to 2 percent of the dry clay weight. Froth flotation with anionic (fatty acid) collectors, selective flocculation by organic polymers and the controlled use of inorganic dispersants are among the wet-processing methods suggested for removing the colored impurities from kaolin clay. "Ultraflotation" (described in U.S. Pat. No. 2,990,958 to E. W. Greene et al) is used on a largescale commercial basis for such purpose. In practicing "Ultraflotation" the colored titaniferous impurities report in a froth along with other metallic impurities and flotation oils. Particulate calcite (added to the flotation feed to aid in flotation of the impurities) is also present in the froth concentrate which has a mustard-like color and an oily plastic consistency. Considering the chemical composition and physical condition of the by-product froth, it is not unexpected that this material has been considered to be a waste effluent, unsuitable without costly preliminary processing as a feed for the production of titania pigments by chlorination or sulfation.
U.S. Pat. No. 3,536,264 to Helton, Jr. et al describes a method for removing titanium mineral impurities from kaolin clays by a modified sedimentation process. The patent suggests that the sediment may be "a valuable source of titanium minerals" but no means for implementing such suggestion are provided. To the best of our knowledge, colored impurities removed by all known kaolin beneficiation techniques are not and have never been upgraded to produce useful articles of commerce.